By user request, version 5 of the MedBetterBUY Chrome extension for use on the eBay site adds two new search listing filters, allowing users to filter out multiple countries and sellers in searches. The upload is now pending release in the Chrome Web Store and should (hopefully) be available soon.
Both filters are accessible from the Configure MedBetterBUY page (not the context menu). If you wish to list multiple countries or sellers, simply separate each by a comma. For example:
Currently the extension only works with the USA eBay site. If you need MedBetterBUY for a different country, let me know and I'll check what would need to be done to add that support.